1. Technical Field
The present invention relates to an organic electroluminescence element (hereinafter also referred to as the xe2x80x9corganic EL elementxe2x80x9d) which utilizes the electroluminescence (hereinafter also referred to as the xe2x80x9cELxe2x80x9d) of organic compounds which emit light in response to a current injected thereinto, and has a light emitting layer formed of a laminate of such materials.
2. Description of the Related Art
Generally, each of the organic EL elements constituting a display panel using organic materials comprises an anode as a transparent electrode, a plurality of organic material layers including an organic light emitting layer, and a cathode comprised of a metal electrode, which are laminated as thin films in this order on a glass substrate as a display surface. The organic material layers include, in addition to the organic light emitting layer, a layer of a material having the hole transport capability such as a hole injection layer and a hole transport layer, a layer of a material having the electron transport capability such as an electron transport layer and an electron injection layer, and so on. Organic EL elements comprising these layers are also proposed. The electron injecting layer also contains an inorganic compound.
As an electric field is applied to the laminate organic EL element including an organic light emitting layer and an electron or hole transport layer, the holes are injected from the anode, while electrons are injected from the cathode. The electrons and the holes are recombined in the organic light emitting layer to form excitons. The organic EL element utilizes light which is emitted when the excitons return to a base state. In some cases, a pigment may be doped into the light emitting layer for improving the efficiency of light emission and stably driving the element.
Recently, it is suggested that phosphorescent materials are used for the light emitting layer of the organic EL element in addition to fluorescent materials. (D. F. O""Brien and M. A. Baldo et al xe2x80x9cImproved energy transfer in electrophosphorescent devicesxe2x80x9d Applied Physics letters Vol. 74 No. 3, pp 442-444, Jan. 18, 1999; M. A. Baldo et al xe2x80x9cVery high-efficiency green organic light-emitting devices based on electrophosphorescencexe2x80x9d Applied Physics letters Vol. 75 No. 1, pp 4-6, Jul. 5, 1999; Tetsuo Tsutsui et al xe2x80x9cHigh quantum efficiency in organic light-emitting devices with Iridium-complex as a triplet emissive centerxe2x80x9d JJAP Vol. 38 (1999) No. 12B in press, pp. L1502-L1504. Generally, luminescence is classified with a criterion in duration of afterglow after termination of the supplied energy into fluorescence at several nano seconds and phosphorescence at a comparative longer period e.g., micro seconds order. The discrimination of fluorescence and phosphorescence is not precise seriously. In phosphorescence, light emission duration is reduced in proportion to elevation of temperature. In fluorescence, light emission duration is not dependent on temperature i.e., afterglow is extremely rapid. In the recombination of electron and hole, electrons drop from an excited state level to the ground state level at which the organic molecule emits light. In this case, the excited organic molecule has the singlet excited state (electrons in inverse spin) of a high energy level and the excited triplet excited state (electrons in the same spin) of a high energy level.
In a study of the organic EL element, organic phosphorescent materials become a focus of attention as a substance to improve the light emission efficiency in recent years. Generally, the light emission process of phosphorescence involves the transition from the ground state to the exited state in the exited molecule, and then the intersystem crossing from the singlet exited state to triplet excited state occurs as a non-emissive transition.
The luminescence process from the triplet excited state to the ground state is usually called phosphorescence. The afterglow process from the triplet excited state through the singlet excited state to the ground state is called delayed fluorescence. The phosphorescence of the organic material is also different in spectrum from an ordinal fluorescence. The phenomenon shows that phosphorescence and fluorescence are different in the excited states (singlet state and triplet state) but the ground state is common. For example, anthracene exhibits phosphorescence of red 670-800 nm wavelengths and fluorescence of blue 470-480 nm wavelengths.
It is expected that a high light emission efficiency caused by the singlet and triplet excited states may be achieved in the utilization of the organic phosphorescence for the light emitting layer in the organic EL element. According to the quantum theory assumption, the ratio of production of singlet and triplet excited states is 1:3 because of difference of spin multiple in the recombination of electron and hole in the organic EL element, which leads that the EL element using phosphorescence may achieve an high light efficiency as three times of the device using fluorescence.
On the other hand, for improving the low power consumption nature, light emission efficiency, and driving stability of the organic EL element, it has been proposed to provide a hole blocking layer between the organic light emitting layer and the cathode for limiting the migration of holes from the organic light emitting layer. Efficient accumulation of holes in the light emitting layer with the aid of the hole blocking layer can result in an improved recombination probability with electrons, and a higher light emission efficiency. A report has been made that single use of a triphenyl diamine derivative or a triazole derivative is effective as a hole blocking material (see Japanese Unexamined Patent Publication Nos. Hei 8-109373 and Hei 10-233284).
While the provisions of an organic phosphorescent material light emitting layer and a hole blocking layer is effective for increasing the light emission efficiency of the organic EL element, a longer lifetime of the element is required. There is a need for a highly efficient organic electroluminescence element which continuously emits light at a high luminance with a less current.
It is an object of the present invention to provide an organic EL element having effectiveness in prolonging life.
An organic electroluminescence element according to the present invention comprises: an anode, a light emitting layer made of an organic compound includes phosphorescent materials, an electron transport layer made of an organic compound and a cathode, which are layered in order, further comprising a hole blocking layer laminated between the light emitting layer and the electron transport layer and made of an aluminium chelate complex represented by the following general formula (1) 
wherein
R2 denotes an alkyl, oxy, amino or hydrocarbon substituent having at least 1 carbon atom, carbon atom being 1 to 10 in any one hydrocarbon moiety,
R3 to R7 independently denote a hydrogen atom, alkyl, oxy, amino or hydrocarbon substituent having at least 1 carbon atom, carbon atom being 1 to 10 in any one hydrocarbon moiety,
R5, R6 and R7 are selected from a group comprises of cyano, halogen, and xcex1-haloalkyl, xcex1-haloalkoxy, amido, sulfonyl, carbonyl, carbonyloxy and oxycarbonyl substituents containing up to 10 carbon atoms,
L denotes one of the following formulae (2) 
xe2x80x83wherein R8 to R12 independently represent hydrogen or hydrocarbon groups of from 1 to 12 carbon atoms, with the proviso that R8 and R9 together or R9 and R10 together can form a fused benzo ring,
R13 to R27 independently represent hydrogen or hydrocarbon groups of from 1 to 12 carbon atoms, with the proviso that R13 and R14 or R14 and R15 can form a fused benzo ring, R18 and R19 or R19 and R20 can form a fused benzo ring, and R23 and R24 or R24 and R25.
In one aspect of the invention, the organic electroluminescence element further includes one or more layers made of a material having a hole transport capability, disposed between said anode and said light emitting layer, said material including an organic compound.
In another aspect of the invention, the organic electroluminescence element further includes one or more mixed layers made of plural kinds of materials having a hole transport capability, disposed between said anode and said light emitting layer, said material including an organic compound.
In a further aspect of the invention, the organic electroluminescence element further includes an electron injecting layer disposed between said cathode and said electron transport layer.
In a still further aspect of the organic electroluminescence element of the invention, said light emitting layer includes, as a main component, an electron transport material having a smaller ionization potential than that of said hole blocking layer.